Cockerels mature at different rates. At some point, if there is a mature hen in the flock, he will try to take control of the flock and be the dominant chicken. Yours is a little unusual in that it is a two-chicken flock but this might be what is going on. I've seen a 5-month-old cockerel do this, I've has some wait until about a year old to try so yours is in that range.
He could be trying to mate. In a part of the mating act the male grabs the back of the hen's head, usually feathers. Sometimes he may mount first, grab the back of the head first, or kind of blend the moves. The head grab instinctively tells the hen to raise her tail feathers up out of the way so he can hit the target. It is instinctive for him to grab, it is instinctive for her to raise her tail. Without the head grab there would be no fertile eggs.
Sometimes the hen resists. If he has a hold on the back of her head he can better control her. The mating act is an act of dominance. The one on top is dominating the one on the bottom. Sometimes this is voluntarily, sometimes it is by force.
Sometimes if she will not accept his dominance, he essentially beats her into submission. They may fight. Sometimes the hen wins, sometimes the male wins. If she does not accept his dominance after he wins she may continue to resist, often by running away. He may continue to attack her at random times until she submits. Or she may continue to attack him until he submits. If he loses, often he will start the cycle over after he matures some more. The male typically wins in the end but it may take a while to get there.
I don't know what is going on with your two. I typically do not interfere unless I see blood or one is injured. It can be pretty violent so injury is possible or they may work it out pretty quickly and be best buddies.
Good luck! It can certainly be hard to watch or it may soon be over.
